The present invention is a fast charging process for lithium-based batteries.
The number and variety of portable devices has been constantly increasing, from small applications such as cellular phones, personal digital assistants (PDAs) and portable computers, to applications as large as electric vehicles and lift trucks. As a result, the focus of improving the performance of these devices has been concentrated in two areas: better batteries, and more energy efficient products. More energy efficient products means computer chips and other components which consume less energy, thereby reducing the frequency of recharging.
The area of better batteries involves two aspects: the battery itself, and the method of charging the battery. With respect to the battery itself, better batteries means smaller and lighter batteries which can store more energy, with a greater energy density, more effectively, for longer periods, under varying conditions of operation, and with more flexibility in packaging. Although battery manufacturers have been successful in developing new batteries that exhibit one or more of these characteristics, those batteries frequently have a shorter cycle-life and use increasingly unstable and less understood elements to achieve those characteristics.
There has also been a strong focus on faster and more efficient methods of charging the battery. However, there are significant differences in the electrochemical natures of different battery types so different types of battery chargers, and different methods of charging, are necessary to address these different battery types and their associated different charging requirements and limitations. However, even with continued advances with respect to more energy efficient devices and better batteries, the users of battery-operated devices continue to experience problems.
Most prior art battery charging methods have focused on charging methods for lead acid, nickel cadmium, and nickel metal hydride batteries. In practice, the majority of chargers in use today for these battery chemistries feature traditional constant current and trickle charging techniques. Others chargers use fast charging techniques which are often nothing more than a higher level of constant current. Some fast charging methods are accompanied by a basic charge termination method, and some use a current cutback as the battery nears the end of charge cycle or approaches a predetermined charge level. Other chargers utilize pulse charging, which consists of positive current pulses which are separated by rest periods, discharge pulses, or both. However, these charging methods often result in long charging times and/or premature battery degradation, and therefore a lack of availability and/or reliability in those battery-operated devices.
Some prior art battery chargers, especially those for use with NiCd and NiMH batteries, measure the temperature of the battery, such as by sensing the resistance of a thermistor which is located inside the battery. If the battery temperature is not within predetermined parameters, a fault condition exists and the charger does not initiate, or stops, the charging process. If there is no fault condition, a rapid charge sequence may be initiated. The temperature and voltage of the battery is monitored. In addition, changes in battery voltage can be monitored. When the slope of the battery-charging curve becomes negative, or the battery temperature reaches a predetermined value, the battery is fully charged, so fast charging is terminated, and a trickle charge process is started.
However, this type of fast charging process does not work well with lithium-based batteries, such as lithium-ion and lithium-polymer batteries, and problems occur when the above fast-charging techniques are used with lithium-based batteries. Continuous high current, i.e., greater than the 1C rate where C is the capacity of the battery, causes metallic lithium to plate or be deposited onto the electrode. This permanently reduces the capacity of the cell. Another problem is the decomposition of the electrolyte. Another problem caused by conventional fast charging methods is overheating of the battery, which causes the battery""s useful life to be shortened. Still another problem is explosive, destructive failure of the battery.
Lithium-ion cells have unique characteristics which make rapid charging difficult: lithium ion cells cannot tolerate the application of a high amplitude direct current. Moreover, lithium ion cells have demonstrated a propensity to explosively fail upon the application of excessive charging voltages. Thus, for safety reasons, all manufacturers impose a voltage limit of approximately 4.2 volts. Further, continuous high current (i.e., greater than the cells 1C rate) causes metallic lithium to plate onto the electrode rather than being adsorbed into the electrode. This can permanently reduce the capacity of the cell.
Thus, for charging lithium-based batteries, a constant current/constant voltage (CC/CV) technique is the most common method. In conjunction with the CC/CV method, many chargers will complete the charge process with a trickle charge stage. If the battery temperature reaches a predetermined value, or the battery voltage reaches some predetermined value, then primary charging is terminated in order to prevent overheating of the battery. The charger is then placed in a trickle-charge mode where the battery is charged at the rate of approximately C/10 to C/20. For example, if the battery has capacity of 1000 mAh, at a C/10 charge rate the charger would charge the battery using a current of 100 mAh. These prior art methods typically require 3 to 10 hours to fully charge a lithium-based battery and still tend to heat the batteries, which causes the battery""s useful life to be shortened.
The long charging time and short battery life result when the concentration gradient increased and the diffusion rate or intercalation of lithium ion into the carbon or graphite electrode decreased and the battery approached a steady state condition wherein the battery would not accept the charging current. When this condition was reached the low amplitude of the charging current resulted in a charging time of hours to complete the charge. When this condition occurred, the charging voltage was often increased so as to force a higher charging current into the battery in an attempt to reduce the charging time. This results in dissolution of the electrolyte, the plating of metallic lithium, and a consequent shortening of the battery life. Thus, in addition to a still too long charging time of 3 to 4 hours, the actual lifetime of those batteries was reduced, generally to about 300 cycles.
Thus, there is a need for a charging technique which provides for safe, fast recharging of lithium-based batteries without causing degradation of the batteries.
The present invention provides a method and an apparatus which safely and rapidly charges lithium-based batteries while reducing the negative side effects which result in premature battery capacity degradation or destructive failure.
In accordance with the preferred embodiment of the present invention, several stages are implemented to effect fast and efficient charging of the battery, full charging of the battery, and terminating the charging of the battery. These stages include a charging stage, two removal stages, and a measurement stage. The charging stage comprises one or more charge pulses separated by rest periods. One removal stage comprises a plurality of alternating charge pulses and discharge pulses, separated by rest periods. Another removal stage comprises one or more large magnitude discharge pulses followed by rest periods. These three stages are applied to the battery in a sequence that rapidly charges the battery, efficiently mixes the electrolyte within the battery, provides voltage and impedance measurements to determine the condition of the battery, and restores a battery""s capacity. The measurement stage controls the application of the other three stages and the parameters of the charge pulses, the discharge pulses, and the rest periods.
The present invention provides for rapid charging of the battery by reducing the build up of a resistive layer (metallic lithium) on the positive electrode, by minimizing dendritic formation which causes internal short circuits, and by minimizing the decomposition of the electrolyte. These three factors, a resistive layer, dendritic formation, and electrolyte 10 decomposition, are generally responsible for decreasing the performance, and therefore the useful life cycle time, of lithium-based batteries.
According to the invention, there is provided a method and algorithm for rapidly charging lithium-based batteries, such as lithium ion and lithium polymer cells and batteries.
The present invention provides a method for charging a battery, comprising the steps of applying a plurality of charge pulses to the battery, the charge pulses each having a duration and being separated by a corresponding plurality of rest periods, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, measuring the charge pulse voltage (CPV) of the battery during the extended charge pulse, applying a subsequent rest period to the battery after the extended charge pulse, measuring the open circuit voltage (OCV) of the battery during the subsequent rest period, determining the difference between the CPV and the OCV, and terminating charging of the battery if the difference is less than a predetermined value.
The present invention also provides another method for charging a battery, comprising the steps of applying a plurality of charge pulses to the battery, the charge pulses each having a duration and being separated by a corresponding plurality of rest periods, applying a plurality of alternating charge pulses and discharge pulses to the battery, measuring the charge pulse voltage (CPV) of the battery during at least one of the alternating charge pulses, applying a subsequent rest period to the battery after the plurality of alternating charge pulses and discharge pulses, measuring the open circuit voltage (OCV) of the battery during the subsequent rest period, determining the difference between the CPV and the OCV, and terminating charging of the battery if the difference is less than a predetermined value. One variation of this method provides for, after the step of applying the plurality of charge pulses, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, and applying a rest period to the battery after the extended charge pulse.
The present invention also provides another method for charging a battery, comprising the steps of applying a plurality of charge pulses to the battery, the charge pulses being separated by a corresponding plurality of rest periods, applying a subsequent discharge pulse to the battery, measuring the loaded circuit voltage (LCV) of the battery during the subsequent discharge pulse, applying a subsequent rest period to the battery after the subsequent discharge pulse, measuring the open circuit voltage (OCV) of the battery during the subsequent rest period, determining the difference between the LCV and the OCV, and terminating charging of the battery if the difference is less than a predetermined value. One variation of this method provides for, after the step of applying the plurality of charge pulses, the charge pulses each having a duration, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, and applying a rest period to the battery after the extended charge pulse. Another variation of this method provides for, after the step of applying the plurality of charge pulses, applying a plurality of alternating charge pulses and discharge pulses to the battery, each discharge pulse having an amplitude, and applying a rest period to the battery after the plurality of alternating charge pulses and discharge pulses, wherein the discharge pulse has an amplitude greater than the amplitudes of the discharge pulses of the plurality of alternating charge pulses and discharge pulses. Still another variation of this method provides for, after the step of applying the plurality of charge pulses, the charge pulses each having a duration, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, applying a rest period to the battery after the extended charge pulse, applying a plurality of alternating charge pulses and discharge pulses to the battery, each discharge pulse having an amplitude, and applying a rest period to the battery after the plurality of alternating charge pulses and discharge pulses, wherein the discharge pulse has an amplitude greater than the amplitudes of the discharge pulses of the plurality of alternating charge pulses and discharge pulses. Still another variation of this method provides for, if the difference is greater than the predetermined value, but less than a second predetermined value, then increasing at least one of amplitude or the duration of the subsequent discharge pulse, and repeating the steps previous to these steps.
The present invention also provides another method of charging a battery, comprising the steps of applying a plurality of charge pulses to the battery, the charge pulses each having a duration and being separated by a corresponding plurality of rest periods, applying a rest period to the battery after the plurality of charge pulses, applying a subsequent discharge pulse to the battery, the subsequent discharge pulse having an amplitude greater than the amplitudes of the plurality of charge pulses, measuring the loaded circuit voltage (LCV) of the battery during the subsequent discharge pulse, applying a subsequent rest period to the battery after the subsequent discharge pulse, measuring the open circuit voltage (OCV) of the battery during the subsequent rest period, determining the difference between the LCV and the OCV, and terminating charging of the battery if the difference is less than a predetermined value. One variation of this method provides for, after the step of applying the rest period to the battery after the plurality of charge pulses, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, the extended charge pulse having an amplitude, and applying a rest period to the battery after the extended charge pulse, wherein the amplitude of the subsequent discharge pulse is greater than the amplitude of the extended charge pulse. Another variation of this method provides for, after the step of applying the rest period to the battery after the plurality of charge pulses, applying a plurality of alternating charge pulses and discharge pulses to the battery, each discharge pulse having an amplitude, applying a rest period to the battery after the plurality of alternating charge pulses and discharge pulses, wherein the subsequent discharge pulse has an amplitude greater than the amplitudes of the discharge pulses of the plurality of alternating charge pulses and discharge pulses. Still another variation of this method provides for, after the step of applying the rest period to the battery after the plurality of charge pulses, applying an extended charge pulse to the battery, the extended charge pulse having a duration greater than the durations of the charge pulses, the extended charge pulse having an amplitude, and applying a rest period to the battery after the extended charge pulse, applying a plurality of alternating charge pulses and discharge pulses to the battery, each discharge pulse having an amplitude, applying a rest period to the battery after the plurality of alternating charge pulses and discharge pulses, wherein the amplitude of the subsequent discharge pulse is greater than the amplitude of the extended charge pulse and greater than the amplitudes of the discharge pulses of the plurality of alternating charge pulses and discharge pulses. Still another variation of this method provides for, if the difference is greater than the predetermined value, but less than a second predetermined value, then increasing at least one of amplitude or the duration of the subsequent discharge pulse, and repeating the steps previous to these steps.
The present invention also provides another method of charging a battery, comprising the steps of applying a discharge pulse to the battery, applying a rest period to the battery after the discharge pulse, applying an extended charge pulse to the battery, applying a rest period to the battery after the extended charge pulse, applying a plurality of alternating charge pulses and discharge pulses to the battery, measuring the loaded circuit voltage (LCV) of the battery during a discharge pulse of the plurality of alternating charge pulses and discharge pulses, applying a rest period to the battery after the plurality of alternating charge pulses and discharge pulses, measuring the open circuit voltage (OCV) of the battery during the rest period after the plurality of alternating charge pulses and discharge pulses, determining the difference between the LCV and the OCV, and terminating charging of the battery if the difference is less than a predetermined value. One variation of this method provides for, prior to the step of applying the discharge pulse to the battery, applying a plurality of charge pulses to the battery, the charge pulses each having an amplitude and being separated by a corresponding plurality of rest periods, the discharge pulse having an amplitude greater than the amplitudes of the plurality of charge pulses. Another variation of this method provides for, prior to the step of applying the discharge pulse to the battery, applying a plurality of charge pulses to the battery, the charge pulses each having an amplitude and being separated by a corresponding plurality of rest periods, the discharge pulse having an amplitude greater than the amplitudes of the plurality of charge pulses, and applying a rest period to the battery after the plurality of charge pulses. Still another variation of this method provides for, if the difference is greater than the predetermined value, but less than a second predetermined value, then increasing at least one of amplitude or the duration of the subsequent discharge pulse, and repeating the steps previous to these steps.
A variation of the above methods which include a plurality of charge pulses provides for increasing a predetermined parameter of the charge pulses of the plurality of charge pulses from the beginning of the plurality to the end of the plurality so that the parameter of the last charge pulse of the plurality of charge pulses is greater than the parameter of the charge pulse of the plurality of charge pulses, wherein the predetermined parameter is a selected one of the amplitude of a the charge pulse or the duration of a the charge pulse.
Another variation of the above methods which include a plurality of charge pulses provides, if the difference is greater than the predetermined value, then increasing the number of the charge pulses of the plurality of charge pulses, and repeating the steps previous to these steps.
Another variation of the above methods which include a plurality of charge pulses provides, if the difference is greater than the predetermined value, but less than a second predetermined value, then increasing the durations of the rest periods between the plurality of charge pulses.
The present invention also provides an apparatus for accomplishing battery charging using the methods described herein.